A series of 2,5-di- and 2,3,4,5-tetraferrocenyl-substituted thiophenes, furans, and pyrroles were synthesized using the Negishi C,C cross-coupling protocol. The electronic and electrochemical properties of these compounds were investigated by cyclic voltammetry (CV), square wave voltammetry (SWV), and in situ UV-vis/NIR spectroscopy. The molecular structures of 2,5-diferrocenyl furan and 2,3,4,5-tetraferrocenyl-1-methyl-1H-pyrrole in the solid state are discussed. The ferrocenyls could sequentially be oxidized giving two or four reversible responses for the appropriate di- or tetraferrocenyl-substituted heterocyclic molecules. The observed ΔE°' values range between 186 and 450 mV. The NIR measurements confirm electronic communication as intervalence charge transfer (IVCT) absorptions were found in the corresponding mono- and in case of the tetraferrocenyl compounds also in the dicationic species. All compounds, except tetraferrocenyl thiophene (a class I system), were classified as class II systems according to Robin and Day. They show a linear relationship between ΔE°' and the IVCT oscillator strength f which could be shown for the first time in organometallic chemistry. This was possible because the series of molecules exhibit analogous geometries and hence, similar electrostatic properties. This correlation was confirmed by electro- and spectro-electrochemical measurements. Within these studies a new approach for the estimation of the effective electron transfer distances r(ab) is discussed.

Planar-chiral ferrocenes are widely applied in catalytic asymmetric transformations in scientific research as well as in industry. A plethora of different methodologies have been developed to access these molecules with a high degree of regio- and stereoselectivity. The aim of this contribution is to give a comprehensive overview of this topic. The synthesis of 1,2- and 1,3-substituted ferrocenes by electrophilic aromatic substitution, ortho-directed metalation, kinetic resolution, and desymmetrization is discussed. Advantages and disadvantages are highlighted.

Novel 2,5-diferrocenyl-1-phenyl-1H-pyrrole (4) and 2,3,4,5-tetraferrocenyl-1-phenyl-1H-pyrrole (6) have been prepared by a 2- or 4-fold Negishi cross-coupling reaction of 2,5-dibromo-1-phenyl-1H-pyrrole (3) and 2,3,4,5-tetrabromo-1-phenyl-1H-pyrrole (5), respectively, with FcZnCl (2) (Fc = Fe(η5-C5H4)(η5-C5H5)) in the presence of [(Ph3P)4Pd] as catalyst. The electronic and structural properties of 4 and 6 were investigated with UV−vis spectroscopy and single-crystal X-ray diffraction (6). Comparison of the appropriate bond distances in the pyrrole core system of 6 demonstrates considerable electron delocalization. Cyclic, square wave, and linear sweep voltammetry as well as in situ NIR spectro-electrochemistry highlight the electrochemical properties of both compounds. Molecules 4 and 6 display two (4) or four (6) electrochemically reversible one-electron transfer processes with remarkably high ΔE 1/2 values and reduction potentials of E 0 ′ = −238 and E 0 ′ = 212 mV for 4 (ΔE 1/2 = 450 mV) and E 0 ′ = −280, E 0 ′ = 51, E 0 ′ = 323, and E 0 ′ = 550 mV for 6 (ΔE 1/2 = 322, 264, and 233 mV) using [NBu4][B(C6F5)4] as the supporting electrolyte. The pyrroles could be classified as class II systems according to Robin and Day. Additionally, 4[PF 6 ] n (n = 1, 2) were synthesized and studied, giving CV responses and NIR spectra identical to those obtained for 4 from electrochemical oxidations.

For the first time the anionic phospho-Fries rearrangement has been successfully applied in ferrocene chemistry, giving access to 1,2-P,O-substituted ferrocenes. The 1,3 (O → C)-migration occurs at ferrocenyl phosphates, thiophosphates, phosphite–borane adducts, and phosphinates by treatment with a base such as lithium diisopropylamide at low temperature, whereas the highest yields were obtained starting from diethylferrocenyl phosphate. Complete reduction of the phosphonate to a primary phosphine and subsequent Stelzer P,C cross coupling allowed the synthesis of Fe(η5-C5H3-2-OMe-PPh2)(η5-C5H5) (1). The qualification of 1 as a supporting ligand in palladium-catalyzed Suzuki–Miyaura C,C couplings has been proven by the synthesis of sterically congested tri-ortho-substituted biaryls under mild reaction conditions in good to excellent yields.

A series of di- and triferrocenyl (hetero)aromatics including 2,5-diferrocenylpyridine (4), 2,6-diferrocenylpyridine (5), 1,3,5-triferrocenylbenzene (9), 2,4,6-triferrocenylpyridine (10), and 2,4,6-triferrocenyl-1,3,5-triazine (11) have been prepared using the Negishi C,C cross-coupling protocol. Characterization of the molecules was performed by spectroscopic means and single-crystal X-ray diffraction study (10). The electrochemical properties of these compounds were determined by cyclic voltammetry, square wave voltammetry, and in situ NIR spectroelectrochemistry. The cyclic voltammograms show well-defined, separated, and reversible one-electron processes for each ferrocenyl moiety with ΔE°′ values (= separation between two formal potentials) ranging from 140 to 185 mV. The UV–vis/NIR spectra of the partially oxidized mixed-valent complexes of 4, 5, 9, and 10 show weak intervalence charge transfer absorptions, while only electrostatic interactions exist between the different oxidized iron centers of 11. Supporting DFT calculations were carried out to establish a basic understanding of the electronic structure.

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A series of Fe(2+) spin crossover (SCO) complexes [Fe(5/6)](2+) employing hexadentate ligands (5/6) with cis/trans-1,2-diamino cyclohexanes (4) as central building blocks were synthesised. The ligands were obtained by reductive amination of 4 with 2,2'-bipyridyl-6-carbaldehyde or 1,10-phenanthroline-2-carbaldehyde 3. The chelating effect and the rigid structure of the ligands 5/6 lead to exceptionally robust Fe(2+) and Zn(2+) complexes conserving their structure even in coordinating solvents like dmso at high temperatures. Their solution behavior was investigated using variable temperature (VT) (1)H NMR spectroscopy and VT Vis spectroscopy. SCO behavior was found for all Fe(2+) complexes in this series centred around and far above room temperature. For the first time we have demonstrated that the thermodynamics as well as kinetics for SCO can be deduced by using VT (1)H NMR spectroscopy. An alternative scheme using a linear correction term C(1) to model chemical shifts for Fe(2+) SCO complexes is presented. The rate constant for the SCO of [Fe(rac-trans-5)](2+) obtained by VT (1)H NMR was validated by Laser Flash Photolysis (LFP), with excellent agreement (1/(kHL + kLH) = 33.7/35.8 ns for NMR/LFP). The solvent dependence of the transition temperature T1/2 and the solvatochromism of complex [Fe(rac-trans-5)](2+) were ascribed to hydrogen bond formation of the secondary amine to the solvent. Enantiomerically pure complexes can be prepared starting with R,R- or S,S-1,2-diaminocyclohexane (R,R-trans-4 or S,S-trans-4). The high robustness of the complexes reduces a possible ligand scrambling and allows preparation of quasiracemic crystals of [Zn(R,R-5)][Fe(S,S-5)](ClO4)4·(CH3CN) composed of a 1 : 1 mixture of the Zn and Fe complexes with inverse chirality.

A series of (oligo)pyrroles featuring redox-active terminal ferrocenyl groups (Fc 2 -( c C 4 H 2 NPh) n (4, n = 1; 9, n = 2; 16, n = 3; 20, n = 4)) has been prepared using a Negishi C,C cross-coupling reaction protocol. The bi-, ter-and quarterpyrrole wire moieties have been built up by C,C cross-coupling reactions of trimethyl silyl protected pyrrole units in the presence of [Pd(CH 2 C(CH 3 ) 2 P( t C 4 H 9 ) 2 )(µ-Cl)] 2 as precatalyst. The structural properties of the title compounds were investigated by spectroscopic means and single-crystal X-ray diffraction studies (9, 16 and 20). The influence of the increasing number of N-phenyl pyrrole units on the electronic interaction between the iron centres was studied using electrochemistry (cyclic (CV) and square wave voltammetry (SWV)) as well as spectroelectrochemistry (in situ UV-Vis/NIR spectroscopy). With exception of the diferrocenyl quarterpyrrol 20, the application of [N n Bu 4 ][B(C 6 F 5 ) 4 ] as electrolyte allows the discrete oxidation of the ferrocenyl termini (∆E° ′ = 450 mV (4), ∆E° ′ = 320 mV (9), ∆E° ′ = 165 mV (16)) in cyclic and square wave voltammograms. However, the iron centres of 20 were oxidized simultaneously, generating dicationic 20 2+ . Additionally, one (9) or two (16 and 20) pyrrole-related well-defined reversible one-electron redox processes were observed. The cyclic voltammetry data reveal that the splitting of the ferrocene-based redox couples, ∆E° ′, decreases with increasing oligo-pyrrole chain length and hence, the larger metalmetal distance. The trends in ∆E° ′ with oligo-pyrrole structure also map to the electronic coupling between the ferrocene moieties as estimated by spectroelectrochemical UV-Vis/NIR measurements.Despite no direct metal-metal interaction in diferrocenyl quarterpyrrole 20, a large absorption in the NIR region is observed arising from photo-induced charge transfer from the oligopyrrole backbone to the redox-active ferrocenyl termini. These charge transfer absorptions have also been found in the dicationic oxidation state of the mono-(4), bi-(9) and terpyrroles (16). Within this series of diferrocenyl(oligo)pyrroles this CT band is shifted bathochromically with increasing chain length of the backbone motif.